As is known in the art, engines have utilized variable cam timing (VCT) mechanisms to control the opening and closing of intake valves and exhaust valves communicating with engine cylinders. In particular, each VCT mechanism is usually utilized to adjust a position of a camshaft (which actuates either intake valves or exhaust valves or both) with respect to a crankshaft position. By varying the position of the camshaft (i.e., camshaft angle) with respect to the position of the crankshaft, engine fuel economy can be increased and engine emissions can be decreased.
One VCT system uses a closed loop system to trim continually between retarding and advancing the camshaft angle to prevent drifting to either the full advance or full retard positions. The sensors used for this closed-loop controller are the Cylinder Identification sensors (CID), and usually of the Variable Reluctance type. Various schemes have been employed in the placement target teeth and the processing of this signal in order to flag when the signals are invalid. When the CID signal is lost or corrupted, a failure flag is set for each CID sensor that is bad.
Various techniques have been suggested to respond to the detection of when a CID tooth signal is invalid (i.e., a missing tooth, a tooth at an incorrect angle, too many teeth after one engine revolution, for example). One technique immediately disables the VCT and resets the camshaft angle to a target, i.e., a predetermined camshaft angle with respect to the position of the crankshaft (sometimes referred to as the base or target timing), during when the failure is detected and immediately re-enables the VCT after failure is gone. The technique is illustrated with reference to FIGS. 1A through 1F. FIG. 1A shows the camshaft angle in the absence of a CID failure. FIG. 1B shows the camshaft angle in response to a single CID failure at time T1 and lasting a relatively short time T2-T1. It is noted that the VCT is disabled during the time from T1 to T2 and is then reactivated at time T2. During the duration between T1 and T2 the camshaft angle to the target, i.e., the predetermined camshaft angle with respect to the position of the crankshaft (sometimes referred to as the base timing). FIG. 1C illustrates a case where there is a low frequency CID error condition and FIG. 1D illustrates where there is a high frequency CID condition. Again it is noted that the VCT is disabled during the time of the CID errors and is then reactivated in the absence of such CID errors. FIGS. 1E and 1F illustrate a low frequency CID error of varying failure duration and a continuous CID error condition, respectively.
Another technique suggested responds to the detection of when a CID tooth signal is invalid is to permanently disable the VCT after an extended period of continuous failure, i.e., after a predetermined failure time threshold level, T. Responses to such technique are illustrated in FIGS. 2A through 2D. It is noted that in FIGS. 2A-2C the CID failure duration is short compared to the predetermined failure time threshold, T. On the other hand, FIG. 2D illustrates a condition where the CID failure duration is greater than the predetermined failure time threshold, T and hence the VCT is disabled after the time T from the CID failure detection.